Background: Magnetic resonance imaging has made it possible to detect asymptomatic lesions of osteonecrosis of the femoral head before abnormalities appear on plain radiographs. The extent of a necrotic lesion is known to be an important prognostic factor. In this study, we evaluated the fate of untreated asymptomatic osteonecrosis of the femoral head with an emphasis on the size of the lesion. We hypothesized that a lesion smaller than a certain size would not progress to symptomatic disease. Methods: One hundred and five initially asymptomatic hips of patients with bilateral nontraumatic osteonecrosis of the femoral head who had been followed without any treatment for at least five years or until pain developed were enrolled in this study. The extent of a lesion was estimated according to the area of the lesion based on a two-dimensional analysis on magnetic resonance images or on plain radiographs at the time of diagnosis. Results: Sixty-two hips became symptomatic, and forty-three hips remained asymptomatic for more than five years (average, eight years and seven months). Of the twenty-one hips with a small necrotic lesion (<30% of the area of the femoral head), one became painful; of the twenty-four hips with a medium-sized necrotic lesion (30% to 50% of the area of the femoral head), eleven became painful; and of the sixty hips with a large necrotic lesion (>50% of the area of the femoral head), fifty became painful. Forty-six of the sixty-two hips that became symptomatic required surgery. Pain developed within five years after the diagnosis in fifty-eight (94%) of the sixty-two symptomatic hips. Conclusions: No treatment appears to be necessary for asymptomatic necrotic lesions with an area smaller than 30% of the femoral head, as the vast majority of these lesions will remain asymptomatic for more than five years. Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.

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31375445

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